From: Bob Liu <lliubbo@gmail.com>
To: Weijie Yang <weijie.yang.kh@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>, Bob Liu <bob.liu@oracle.com>,
Seth Jennings <sjenning@linux.vnet.ibm.com>,
Linux-MM <linux-mm@kvack.org>,
Linux-Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [BUG REPORT] ZSWAP: theoretical race condition issues
Date: Wed, 25 Sep 2013 18:02:15 +0800 [thread overview]
Message-ID: <CAA_GA1eBS642zSsAd8W=n6oAD7hpKumXTneoSSMqWZxyr4QWng@mail.gmail.com> (raw)
In-Reply-To: <CAL1ERfOqoo+tPNYQn+e=pqP761gk+bAd7AyeXfoxogfNy0N6Lg@mail.gmail.com>
On Wed, Sep 25, 2013 at 5:33 PM, Weijie Yang <weijie.yang.kh@gmail.com> wrote:
> On Wed, Sep 25, 2013 at 4:31 PM, Bob Liu <lliubbo@gmail.com> wrote:
>> On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang <weijie.yang.kh@gmail.com> wrote:
>>> I think I find a new issue, for integrity of this mail thread, I reply
>>> to this mail.
>>>
>>> It is a concurrence issue either, when duplicate store and reclaim
>>> concurrentlly.
>>>
>>> zswap entry x with offset A is already stored in zswap backend.
>>> Consider the following scenario:
>>>
>>> thread 0: reclaim entry x (get refcount, but not call zswap_get_swap_cache_page)
>>>
>>> thread 1: store new page with the same offset A, alloc a new zswap entry y.
>>> store finished. shrink_page_list() call __remove_mapping(), and now
>>> it is not in swap_cache
>>>
>>
>> But I don't think swap layer will call zswap with the same offset A.
>
> 1. store page of offset A in zswap
> 2. some time later, pagefault occur, load page data from zswap.
> But notice that zswap entry x is still in zswap because it is not
Sorry I didn't notice that zswap_frontswap_load() doesn't call rb_erase().
> frontswap_tmem_exclusive_gets_enabled.
> this page is with PageSwapCache(page) and page_private(page) = entry.val
> 3. change this page data, and it become dirty
> 4. some time later again, swap this page on the same offset A.
>
> so, a duplicate store happens.
>
Then I think we should erase the entry from rbtree in zswap_frontswap_load().
After the page is decompressed and loaded from zswap, still storing
the compressed data in zswap is meanless.
--
Regards,
--Bob
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next prev parent reply other threads:[~2013-09-25 10:02 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-18 16:14 Weijie Yang
2013-08-19 2:17 ` Bob Liu
2013-08-19 5:47 ` Minchan Kim
2013-08-20 15:22 ` Weijie Yang
2013-08-21 12:11 ` Bob Liu
2013-08-20 15:30 ` Weijie Yang
2013-08-21 7:49 ` Minchan Kim
2013-09-25 8:09 ` Weijie Yang
2013-09-25 8:31 ` Bob Liu
2013-09-25 9:33 ` Weijie Yang
2013-09-25 10:02 ` Bob Liu [this message]
2013-09-26 2:06 ` Weijie Yang
2013-09-26 5:58 ` Minchan Kim
2013-09-26 7:26 ` Weijie Yang
2013-09-26 7:57 ` Minchan Kim
2013-09-26 8:48 ` Weijie Yang
2013-09-26 9:56 ` Bob Liu
2013-09-27 4:58 ` Minchan Kim
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